1. Field of the Invention
The invention relates to a bus communication system, and in particular to a bus communication system in which a hub device is provided between a host and a bus device under the universal serial bus (USB) or USB-OTG (USB-On-The-Go) standard and communication is made between the host and the bus device over tiered hub devices. The invention relates also to a line-concentrating and switching device advantageously for use in a hub device, which is provided in tiers for transmitting data. This invention further relates to a host device to control the hub device and devices, which are interconnected in tiers to form a network.
Furthermore, the invention relates to a communication control method for a bus communication system, which is in keeping with the function and topology of hub and bus devices provided in tiers, and in particular to a communication control method advantageously applicable to a multi-stage connection of bus devices under the USB or USB-OTG standard. Finally, the invention relates to a communication control method of a network system composed of plural, interconnected networks under different standards, in which communication may be made from one network to a device connected in a lower layer of another network. The present invention may be applied with advantage to for example a communication control method in a network system where there co-exist networks of the USB-OTG and USB standards.
2. Description of the Background Art
Nowadays, USB devices have become widespread as a simplified peripheral of an personal computer, while the USB has come to be well-known as an interface which takes the place of, for example, the RS-232C (Recommended Standard-232C) which is a conventional serial interface standard. The USB standard was initially of the USB version 1.0 and later updated to the USB version 1.1. Still later, the USB standard has been updated to version 2.0, in which the transmission rate is raised from conventional 12 Mbit/sec to 480 Mbit/sec in keeping with the increasing operating speed of peripherals.
First, the USB is featured by a low cost. Second, the USB is featured by allowing for freedom in layout. Specifically, the USB is able to cope with the plug and play function, which automatically recognizes peripheral connected to allow for use without setting a driver. Additionally, the USB has a hot plug function, which allows plug-in and plug-out of a peripheral without turning off the power supply. In a layout aspect, the hub device may be applied for interconnecting a terminal device in the LAN (Local Area Network).
Japanese Patent Laid-Open Publication No. 2002-202835 proposes a peripheral electronic device, which is premised on a tree connection topology and may be directly connected to another peripheral electronic device similarly premised on a tree connection topology through the same connector. The proposed electronic device checks the state of connection resulting from the coupling of the connector to verify to which the connector is connected, in order to perform an operation accordingly.
Since the USB standard allocates seven bits to the address of a bus device, 127 USB devices at the maximum can be connected, in addition to a host device. However, hub devices and USB cables suffer from delay time. In consideration of delay time, the tree connection structure, with a host device located at an apex point, is limited to seven tiers. Specifically, if a host device presents in the first layer, the USB devices are of such a structure in which up to the seventh tier is allowed for the terminal USB device. However, the USB prescriptions lack for a statement that the connection structure is to be limited to seven tiers, and simply provide for delay time of USB devices, etc.
It is explicitly stated that delay time from the upstream to downstream end connector of a hub device shall be 70 nanosecond or less, and that a signal propagation delay in one direction shall be 30 nanosecond or less. In addition, it is explicitly stated that a cable used shall have attenuation characteristics of 5.8 dB or less per cable at a frequency of 400 MHz.
There is disclosed in Japanese Patent Laid-Open Publication No. 2000-194649 a method for connecting electronic equipment of a tree connection topology directly to each other without contravening the USB standard. In this connection method, a device of the USB standard is used in common by plural host devices satisfying the USB standard. The plural host devices are connected to plural high-end ports of a hub device, as a connection device for electronic equipment, to plural low-end ports to which devices of the USB standard are connected. One of the high-end ports and one of the low-end ports are selected to operate the host device and the device of the USB standard associated with the selection. There is no necessity for detaching and connecting again the cables, thus improving user friendliness.
There is also disclosed in Japanese Patent Laid-Open Publication No. 2002-288112 an interface system and an interface device in which the USB standard is satisfied and a free network system may be constructed in such a manner as to override the constraint imposed by the standard. In order to realize this, a semiconductor device for communication control is featured by host controlling means and function controlling means which manage communication control as a slave, with the host controlling means and the function controlling means mounted on a single semiconductor chip and being in operation simultaneously. The interface system includes a semiconductor device for communication control, a first connector connectable to host equipment, a second connector connectable to function equipment, and an external switching means connected between the first and second connectors and the transmitting/received data input and output terminals of the semiconductor device for communication control. The external switching means are controlled in an operatively interlocked relation to the switching means provided within the semiconductor device for communication control, in order to connect the host equipment or the function equipment at all times to the two connectors, whereby data may be transmitted and received between the devices without changing the cable connection.
It should be noted that the delay time such as encountered with the above-mentioned USB standard limits the turn-around time of a USB bus. The turn-around time is provided for preventing signal collision on the bus. Thus, until a device of the USB standard has received a packet from a host device and the turn-around time elapses, the device of the USB standard cannot send out a packet to the host device. This turn-around time is requested to secure when devices of the USB standard communicate with each other.
The USB standard provides that the cable length in general shall be five meters at the maximum. Thus, the total cable length from a host device to a terminal device of the USB standard in the lowermost layer amounts to 30 meter. However, the USB standard simply states that the length shall be limited by the aforementioned attenuation characteristics and the signal propagation time. Such USB prescriptions are presumably intended to provide for flexibility in order to take into account the possible future improvement in cables, etc. However, rather detailed prescriptions are provided in connection with connectors in view of securing the compatibility.
With USB devices, the bus topology takes a so-called star connection. Between a host device of the first tier and a USB device located at the lowermost layer or the seventh tier, there are five hub devices, that is, five tiers. If a number of USB devices corresponding to the connectable limit are connected to a sole hub device, the connection topology may be likened to that of placing many loads to a sole electrical outlet. In actual, if tiers are formed using one, or at most two, hub devices, with a personal computer, placed in a home, as a host, plural USB devices, such as a keyboard, a mouse, a printer, a scanner, a terminal adaptor (TA) or an external storage unit, are connected to the tiers in a disorderly fashion. Even granting that up to 127 USB devices can be connected under the USB standard, an extremely difficult condition emerges with the wiring necessary for the connection.
If, in order to improve the difficult condition, plural USB devices are connected in series and connection is made in such a manner as to override the seven-tier limitation, the result is a neat connection topology. If, for example, connection is made to a tier beyond the limitation on the number of tiers, the number of USB devices connected per tier may be reduced, such that the number of the ports of the topology may be made approximately equal to the number of the ports in the above-mentioned difficult condition. Specifically, the resulting connection is desirable for a case in which a large number of USB sensors are arranged in a wide area and measurement is made on the sensors by a personal computer operating as a host device. In this case, a portable USB device is desirably used as the USB sensor. However, in order to make connections in this topology and meet the aforementioned USB prescriptions, it is requested to solve the problem of the delay time of USB devices and/or the turn-around time of USB buses.
Recently, there is proposed the USB-OTG standard as an architecture of constructing a more flexible network using devices of the USB standard. This architecture, which employs an interface establishing peer-to-peer connection between USB devices without the mediation of a host device, is effective to moderate several difficult conditions.
The first difficulty is the increased load in adding a function owned by the host device to the aforementioned USB portable device. This increased load is caused by providing a USB portable device with a storage area for storing plural device drivers. The second difficulty is the necessity of supplying a large current to a USB portable device, specifically, the use of a high-efficiency power supply. The third difficulty is the physical load imposed by employing an “A” connector for the host device.
With the USB-OTG standard, the function as specified in connection with the first difficulty in communication between USB portable devices is not needed. Consequently, the standard that slightly complements the aforementioned standard of the USB Version 2.0 (On-The-Go Supplement to the USB 2.0 Specification) was formulated in the year of 2001.
The connection between the first and second networks under the USB-OTG standard will be described. Before this description, the connection topology of the first network in the case of using the USB standard will be described. In the first network, a PDA (personal digital assistant) working as a host device is connected by a USB cable to a hub device, which is connected through one of the ports owned by the hub device and a USB cable to a printer. It is now assumed that a personal computer as a host device is newly connected to the hub device and the printer is controlled from this personal computer in order to use the printer. The newly added personal computer is connected through a port of the hub device and four-conductor USB cable. However, the personal computer is unable to directly communicate with the printer, even though it is attempted to use the printer over a hub device. The reason is that the PDA is designed to supervise the network including the PDA, hub device and printer in accordance with the USB standard, in other words, that the first network in the state of art is not constructed in accordance with the USB-OTG standard.
The newly added personal computer is connected to the PDA as a constituent element of the second network. The USB-OTG standard is applied to the PDA and the personal computer used in the second network. The respective OTG ports of the PDA and the personal computer are connected to the five-conductor OTG cable. In this case, the communication is possible between the personal computer and the PDA on the network. However, the personal computer on the second network cannot utilize the printer in the first network. The reason is that, while the USB-OTG standard allows communication in the second network, specifically, communication between the personal computer and the PDA, the printer in the first network operating in the USB standard communicates only with the PDA of the host device.